Europeans were reeling Sunday from President Trump's announcement that eight countries will face a 10% tariff for opposing American control of Greenland.

The responses to Mr. Trump's decision ranged from saying it risked "a dangerous downward spiral" to predicting that "China and Russia must be having a field day."

Mr. Trump's threat sets up a potentially dangerous test of U.S. partnerships in Europe. Several European countries have sent troops to Greenland in recent days, saying they are there for Arctic security training. Mr. Trump's announcement came Saturday as thousands of Greenlanders were wrapping up a protest outside the U.S. Consulate in the capital, Nuuk.

The Republican president appeared to indicate that he was using the tariffs as leverage to force talks with Denmark and other European countries over the status of Greenland, a semiautonomous territory of NATO ally Denmark that he regards as critical to U.S. national security. Denmark, Norway, Sweden, France, Germany, the United Kingdom, the Netherlands and Finland would face the tariff.

The eight countries issued a joint statement Sunday: "As members of NATO, we are committed to strengthening Arctic security as a shared transatlantic interest. The pre-coordinated Danish exercise 'Arctic Endurance,' conducted with Allies, responds to this necessity. It poses no threat to anyone."

The statement added: "We stand in full solidarity with the Kingdom of Denmark and the people of Greenland. Building on the process begun last week, we stand ready to engage in a dialogue based on the principles of sovereignty and territorial integrity that we stand firmly behind. Tariff threats undermine transatlantic relations and risk a dangerous downward spiral. We will continue to stand united and coordinated in our response. We are committed to upholding our sovereignty."

There are immediate questions about how the White House could try to implement the tariffs, because the EU is a single economic zone in terms of trading. Norway and the U.K. are not part of the 27-member EU, and it was not immediately clear if Mr. Trump's tariffs would impact the entire bloc. EU envoys scheduled emergency talks for Sunday evening to determine a potential response.

It was unclear, too, how Mr. Trump could act under U.S. law, though he could cite emergency economic powers that are currently subject to a Supreme Court challenge.

European Union foreign policy chief Kaja Kallas said China and Russia will benefit from the divisions between the U.S. and Europe. She added in a post on social media: "If Greenland's security is at risk, we can address this inside NATO. Tariffs risk making Europe and the United States poorer and undermine our shared prosperity."

Mr. Trump's move also was panned domestically.

Sen. Mark Kelly, a former U.S. Navy pilot and Democrat who represents Arizona, posted that Mr. Trump's threatened tariffs on U.S. allies would make Americans "pay more to try to get territory we don't need."

"Troops from European countries are arriving in Greenland to defend the territory from us. Let that sink in," he wrote on X. "The damage this President is doing to our reputation and our relationships is growing, making us less safe. If something doesn't change we will be on our own with adversaries and enemies in every direction."

A CBS News poll released Sunday found widespread opposition among Americans to buying Greenland or taking it by military force. Seventy percent said they would oppose using federal funds to buy the territory, and 86% said they would oppose seizing it militarily.

The tariffs announcement even drew blowback from Mr. Trump's populist allies in Europe.

Italy's right-wing Premier Giorgia Meloni, considered one of Mr. Trump's closest allies on the continent, said Sunday she had spoken to him about the tariffs, which she described as "a mistake."

The deployment to Greenland of small numbers of troops by some European countries was misunderstood by Washington, Meloni told reporters during a two-day visit to South Korea. She said the deployment was not a move against the U.S. but aimed to provide security against "other actors" that she didn't name.

Jordan Bardella, president of Marine Le Pen's far-right National Rally party in France and also a European Parliament lawmaker, posted that the EU should suspend last year's tariff deal with the U.S., describing Mr. Trump's threats as "commercial blackmail."

Mr. Trump also achieved the rare feat of uniting Britain's main political parties — including the hard-right Reform UK party — all of whom criticized the tariff threat.

"We don't always agree with the U.S. government and in this case we certainly don't. These tariffs will hurt us," Reform UK leader Nigel Farage, a longtime champion and ally of Mr. Trump, wrote on social media. He stopped short of criticizing Mr. Trump's designs on Greenland.

Meanwhile, U.K. Prime Minister Keir Starmer, who leads the center-left Labour Party, said the tariffs announcement was "completely wrong" and his government would "be pursuing this directly with the U.S. administration."

The foreign ministers of Denmark and Norway are also expected to address the crisis Sunday in Oslo during a news conference.